Still time to catch sight of elusive Mercury

The planet Mercury is often cited as the most difficult of the five bright planets to see. Called an "inferior planet" because its orbit is nearer to the sun than Earth's, Mercury — scarcely more than half as far from the sun as Venus is — always appears from our vantage point to be in the same general direction as the sun and is usually lost in the sunlight.

Three times a year, this rocky little world emerges into the evening twilight for a few weeks, and at three other times in the year it ventures a little way into the morning sky. Yet, even at these "greatest elongations," it can't be easily seen unless other conditions are favorable. It is important, for instance, that Mercury be positioned as nearly directly above the sun as possible — a condition that is best fulfilled just after sunset in the spring and before sunrise in autumn. [June Night Sky: Visible Planets, Moon Phases & Events]

For those living in the Northern Hemisphere, a great window of opportunity for viewing Mercury in the evening sky has just opened up. That window, in fact, will remain open through at least June 21, giving you an ample number of chances to see this so-called "elusive planet" with your own eyes.

Use Venus to find MercuryCurrently, Mercury is visible about 45 minutes after sunset. You can use the brilliant planet Venus to guide you to Mercury; an artifact of last month's "Great Conjunction."

Just look low toward the west-northwest horizon and you'll readily see Venus shining brightly in the twilight sky. Then look about 5 degrees above and to the left of Venus. (Your clenched fist held at arm's length measures 10 degrees, so the much fainter Mercury will be a "half fist" from Venus. Mercury will be shining with just a trace of a yellowish-orange tinge.

In fact, Mercury will be shining at a very respectable magnitude of +0.2; almost as bright as Capella and Vega, two of the brightest stars in the sky. However, Venus will appear more than 60 times brighter than Mercury!

In the evenings that follow, Mercury will slowly diminish in brightness, but it will also slowly gain altitude as it gradually moves away from the vicinity of the sun.

Moon joins the showLow toward the western horizon just after sunset on Monday evening, a lovely, wire-thin crescent moon shone, about 2.5 days after passing through new moon phase and only 5 percent illuminated. About 7 degrees above and to the right of that slender sliver was a bright "star" shining in the twilight.

That star is the elusive planet Mercury

Mercury arrived at its greatest elongation two nights later on Wednesday, when it was 24 degrees to the east of the sun. By then, the moon had moved well away from Mercury and to its upper left. Shining on this night at magnitude 0.6 (appearing just a bit brighter than the star Betelgeuse, in the constellation of Orion), Mercury set one hour and 45 minutes after the sun, making this an excellent evening apparition.

Mercury, like Venus, appears to go through phases like the moon. When June began, its disk was 60 percent illuminated by the sun, giving it a gibbous appearance in telescopes, which is also why it started out the month appearing noticeably brighter than it is now.

By the time of its greatest elongation, it appeared 37 percent illuminated, and the amount of its surface illuminated by the sun will continue to decrease in the days that follow. Now that it has begun to turn back toward the sun's vicinity, it has started fading.

Venus hangs aroundBut Venus will continue to guide viewers toward Mercury as it will press closer during the following nights. On June 18, the two planets will appear side-by-side, with Mercury on the left and Venus on the right. They'll be separated by just 2.5 degrees. They'll be even closer — less than 2 degrees apart — the next night on June 19, when Mercury will be to the lower left of Venus.

In fact, by the evening of June 21, Mercury's brightness will have dropped to magnitude +1.4, slightly dimmer than the star Regulus in Leo; only one-third as bright as it was on June 7.

In telescopes, Mercury will appear in its narrowing crescent phase. Thus, in all likelihood, June 21 will be one of your last views of it. The combination of its lowering altitude, plus its descent into a much-brighter sunset glow, should finally render Mercury invisible by the final week of June. It will pass through inferior conjunction — between the sun and Earth — on July 9.

Dual identitiesIn old Roman legends, Mercury was the swift-footed messenger of the gods.

The planet is well named for it is the closest planet to the sun and the swiftest of the sun's family, averaging a speed of about 30 miles per second, and making its yearly journey in only 88 Earth days.

Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experience periods of intense heat and extreme cold. Although its mean distance from the sun is only 36 million miles, Mercury experiences by far the greatest range of temperatures: nearly 900 degrees Fahrenheit (482 degrees Celsius) on its day side; minus 300 F (minus 184 C) on its night side.

In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the sun and then the other. Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, in about the fifth century B.C., pointed out that they were one and the same.

Editor's note: If you snap an amazing photo of Mercury in the night sky, or any other celestial object, and you'd like to share for a possible story or image gallery, please send images and comments, including location information, to Managing Editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us on Twitter, Facebook and Google+. Original article on Space.com.